Once someone says “Tasmota,” as the OP does in this case, it means they have flashed the firmware of the device and replaced it with the Tasmota firmware. At that point none of the original Shelley docs will apply.
Use of the Tasmota software was popular in the community for tech savvy people who wanted to use a Wi-Fi device to communicate directly with the smartthings cloud, rather than going through the manufacturer’s cloud.
I know the OP wanted to use the MQTT method, but we should probably also mention that @hongtat does have an edge Driver for Tasmota-flashed devices which covers literally thousands of models.